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Browsers have security policies in place to stop this exact thing from being possible. You can ask your clients for this information, but I highly doubt it most people will just hand over such information without a solid reason why.

Java, on the other hand, is enabled by a lot of people and will happily feed back all sorts of info like the local IP address (not the proxy server, gateway, etc...) and may possibly have access to the information you want as well.